Travis: add a build check for the GH Pages site

Adding a stage to check on each PR and merge to `master` whether the GH pages site can be build without errors.

Author: jrfnl

.travis.yml

@@ -1,3 +1,4 @@
+os: linux
 dist: trusty
 
 language: php
@@ -10,6 +11,7 @@ cache:
     - $HOME/.composer/cache/files
     # Cache directory for more recent Composer versions.
     - $HOME/.cache/composer/files
+    - /home/travis/.rvm/
 
 php:
   - 5.5
@@ -31,6 +33,8 @@ env:
 # See: https://docs.travis-ci.com/user/conditions-v1
 stages:
   - name: sniff
+  - name: docs
+    if: branch IN (master)
   - name: quicktest
     if: type = push AND branch NOT IN (master, develop)
   - name: test
@@ -66,6 +70,24 @@ jobs:
         # Check the code-style consistency of the xml files.
         - diff -B ./PHPCSUtils/ruleset.xml <(xmllint --format "./PHPCSUtils/ruleset.xml")
         - diff -B ./PHPCS23Utils/ruleset.xml <(xmllint --format "./PHPCS23Utils/ruleset.xml")
+      after_success: skip
+
+    #### DOCUMENTATION SITE TESTING STAGE ####
+    - stage: docs
+      language: ruby
+      cache: bundler
+      rvm:
+        - 2.5.8
+      before_install: skip
+      install:
+        - cd ./docs
+        - gem install bundler
+        - bundle install
+      before_script: skip
+      script:
+        # Test the documentation generation.
+        - bundle exec jekyll build
+      after_success: skip
 
     #### QUICK TEST STAGE ####
     # This is a much quicker test which only runs the unit tests and linting against the low/high